Resonant effects in dielectric gratings were identified in the early 1990's as having promising applications to free-space optical filtering and sensing. Resonant effects typically occur in sub-wavelength gratings, where the first-order diffracted mode corresponds not to freely propagating light but to a guided wave trapped in some dielectric layer. The trapped wave is re-scattered in the 0th diffracted order and interfaces with the incident light to create a pronounced modulation of transmission and reflection. When a high-index-contrast grating is used, the guided waves are rapidly scattered and do not propagate very far laterally. As a result, the resonant feature can be considerably broadband and of high angular tolerance, which has been used to design novel types of highly reflective mirrors. Recently, sub-wavelength grating mirrors have been used to replace the top dielectric stacks in vertical-cavity surface-emitting lasers, and in novel micro-electromechanical devices. In addition to being more compact and relatively cheaper to fabricate, sub-wavelength grating mirrors also provide polarization control.
Although in recent years there have been a number of advances in sub-wavelength gratings, designers, manufacturers, and users of optical devices continue to seek grating enhancements that broadening the possible range of grating applications.